Though 300 amino acids occur in nature, about 20 amino acids are found in proteins. All the amino acids are required by the body but some are known as indispensable nutritionally essential amino acids, as these cannot be synthesized in the body, and their deficiency disturbs nitrogen equilibrium, growth, nutrition, maintenance, and life span. The others are termed as dispensable in the diet as there amino acids can be synthesized in the body, but they are more important to the cell than the essential ones. There are 10 essential amino acids in humans.

Arginine, Histidine, Isoleucine, Leucine, Lysine, Methionine, Phenylalanine, Threonine, Tryptophan, and Valine. Arginine and Histidine are considered nutritionally semiessential, since they can be synthesized but not in quantities sufficient to permit normal growth. Two other amino acids, Cysteine and Tyrosine may be formed form the essential amino acids Methionine and Phenylalanine, respectively.

The others are Alanine, Asparagine, Aspartate, Glutamate, Glutamine, Glycine, Proline, and Serine. These are formed from amphibolic intermediates by short anabolic pathways, or from other dietary amino acids.

Proteins form different sources vary considerably in the quality and quantity of their amino acid contents, and thus their food values differ. The building blocks of proteins are amino acids. While proteins contain substances in addition to amino acids, their biologic properties are determined largely by the kinds of amino acids, present, the order in which they are linked together, and thereby the spatial relationship of one amino acid to another.

First class proteins of high biological value contain all essential amino acids and are capable of providing growth and maintenance. Almost all animal proteins, glutein of wheat, and glutelin of maize belong to this class. Second class proteins of low biological value are different in some of the essential amino acids and unsuitable for growth and maintenance. Generally most of the vegetable origin proteins, and gelatin of animal origin fall in this category. A diet supplying 12% energy from protein in adequate.

Certain amino acids, like Glycine and Glutamic acid, appear to be involved in the transmission of impulses in the nervous system. Know more about 20 amino acids.

The metabolism of amino acids rise to many compounds of biomedical importance: as decarboxylation of certain amino acids produces the corresponding amines like Histamine and Gama-aminobutyric acid (GABA) having biologic functions.

The protein values in food items are calculated from the functions, content, and the factor used is 6.25 for all foodstuffs. The amount of nitrogen in gm per 100 gm of edible portion of a foodstuff is given in col. 2 of table below. The col. 3 to 14 of the table further indicate content of 12 amino acids in mg per gm of nitrogen. So to calculate:

Protein =N (col.2) x6.25 in gm per 100 gm of edible portion of foodstuff. Amino acid= N x 6.25xAmino Acid (any of col. 3-14) in mg in 100 gm of food.